The present invention relates to an improved scale balancing device incorporated into a scale manipulating head of the universal parallel ruler (drawing machine).
In the scale manipulating head of the drawing machine, an arrangement is generally provided such that the scale rotating shaft is rotatably supported in the bearing of the scale supporting base plate. The scale rotating shaft has the scale manipulating handle and the scale mounting plate both formed integrally therewith at the upper end and the lower end, respectively, so that said scale manipulating handle may be used to rotate the scale. In such an arrangement, due to the fact that the scale extends in one direction from its rotating shaft in a cantilever fashion, an empty weight or dead load of the scale produces an unbalanced load which is disadvantageously applied to the scale rotating shaft so far as the scale is free to be rotated, when the drawing board is installed at an angle. A rotating moment due to such empty weight of the scale is zero when the center of gravity of the scale is vertically just above the manipulating handle and when said center of gravity is vertically just below said manipulating handle while said rotating moment gradually increases as the center of gravity of the scale is rotated from siad position just above the manipulating handle to the horizontal and at this moment said rotating moment reaches its maximum value. As said center of gravity is further rotated from the horizontal to said position vertically just below the manipulating handle, said rotating moment due to the empty weight gradually decreases down to zero. Accordingly, the scale would be unintentionally rotated downwardly due to said empty weight of the scale unless the scale is positively supported by the scale manipulating handle. It is an essential object of the present invention to balance such rotating moment produced due to the empty weight of the scale so that the scale may be held at any desired angular position and the rotational manipulation of the scale may be smoothly achieved, by providing an improved scale balancing device.
Such a prior art device is disclosed by publications such as Japan Unexamined Patent Publication No. 59-179400 and Japan Unexamined Patent Publication No. 60-149496.
In this device of well known art, in order to avoid the inconvenience that, when the rotating moment developed by the empty weight of the scale is balanced by a balancing mechanism concentrically mounted on the scale rotating shaft, an unbalanced load is exerted on the scale rotating shaft to produce an error in the scale rotation angle, the first rotatable member adapted to be rotated concentricallty with the scale rotating shaft is coupled through the interlocking mechanism to the second rotatable member separately provided on the non-rotatable member of the scale manipulating head so that the first and second rotatable members may be rotated synchronously with each other and thereby a tensile force against the rotating moment due to the empty weight of the scale is exerted on said second rotatable member in order to maintain the scale well balanced.
With this prior art device, however, the design of the non-rotatable member included in the head of the conventional universal parallel ruler must be substantially modified, since the second rotatable member of the same size as the first rotatable member must be incorporated into the non-rotatable member. This prior art device is accompanied with further problems in that the non-rotatable member inevitably becomes bulky, it is impractical to perfectly remove the unbalanced load in order to balance the rotating moment due to the empty weight of the scale, and finally the interlocking mechanism serving to achieve a synchronoue rotation of the first and second rotatable members causes an error in the rotational angle of the scale rotating shaft.